The present invention relates generally to nonreversible metal hydrides used to store and release hydrogen. More particularly the present invention relates to nonreversible metal hydrides as a type of hydrogen fuel for use in a variety of design applications.
As the world""s population expands and its economy increases, the atmospheric concentrations of carbon dioxide are warming the earth causing climate change. However, the global energy system is moving steadily away from the carbon-rich fuels whose combustion produces the harmful gas. For nearly a century and a half, fuels with high amounts of carbon have progressively been replaced by those containing less.
In the United States, it is estimated, that the trend toward lower-carbon fuels combined with greater energy efficiency has, since 1950, reduced by about half the amount of carbon spewed out for each unit of economic production. Thus, the decarbonization of the energy system is the single most important fact to emerge from the last 20 years of analysis of the system. It had been predicted that this evolution will produce a carbon-free energy system by the end of the 21st century. The instant invention helps to greatly shorten that period. In the near term, hydrogen will be used in fuel cells for cars, trucks and industrial plants, just as it already provides power for orbiting spacecraft. But ultimately, hydrogen will also provide a general carbon-free fuel to cover all fuel needs including power generation.
In the 1800s, fuel was primarily wood in which the ratio of hydrogen to carbon was about 0.1. As society switched to the use of coal and oil, the ratio of hydrogen to carbon increased first to 1.3 and then to 2. Currently, society is inching closer to the use of methane in which the hydrogen to carbon ratio is further increased to 4 (methane has serious problems with safety, cost and infrastructure). However, the ultimate goal for society is to employ a carbon-free fuel, i.e., the most ubiquitous of elements, pure hydrogen. The obstacle has been the lack of solid state storage capacity and infrastructure.
Hydrogen is the xe2x80x9cultimate fuel.xe2x80x9d It is inexhaustible. Hydrogen is the most plentiful element in the universe (over 95% of all matter). Hydrogen can provide a clean and sustainable source of energy for our planet and can be produced by various processes which split water into hydrogen and oxygen. The hydrogen can then be stored and transported in solid state form.
While the world""s oil reserves are depletable, the supply of hydrogen remains virtually unlimited. Hydrogen, which can be produced from coal, natural gas and other hydrocarbons, is preferably formed via electrolysis of water, more preferably using energy from the sun (see U.S. Pat. No. 4,678,679, the disclosure of which is incorporated herein by reference.) However, hydrogen can also be produced by the electrolysis of water using any other form of economical energy (e.g., wind, waves, geothermal, hydroelectric, nuclear, gasification of biomass, etc.) Furthermore, hydrogen, is an inherently low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of xe2x80x9cburningxe2x80x9d hydrogen is water. Thus, hydrogen can be a means of solving many of the world""s energy related problems, such as climate change, pollution, strategic dependency on oil, etc., as well as providing a means of helping developing nations in sustainable manner.
While hydrogen has wide potential application as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of an acceptable lightweight hydrogen storage medium. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. Additionally, large and very expensive compressors are required to store hydrogen as a compressed gas and compressed hydrogen gas is a very great explosion/fire hazzard.
Hydrogen also can be stored as a liquid. Storage as a liquid, however, presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below xe2x88x92253xc2x0 C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen. Another drawback to storage as a liquid is the costly losses of hydrogen due to evaporation, which can be as high as 5% per day.
Hydrogen can be stored in a solid hydride and can provide a greater percent weight storage than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride is safe and does not present any of the hazard problems that hydrogen stored in containers as a gas or a liquid does, because hydrogen, when stored in a solid hydride form, exists in its lowest free energy state.
In addition to the problems associated with storage of gaseous or liquid hydrogen, there are also problems associated with the transport of hydrogen in such forms. For instance, transport of liquid hydrogen will require super-insulated tanks, which will be heavy and bulky and will be susceptible to rupturing and explosion. Also, a portion of the liquid hydrogen will be required to remain in the tanks at all times to avoid heating-up and cooling down of the tank which would incur big thermal losses. As for gaseous hydrogen transportation, pressurized tankers could be used for smaller quantities of hydrogen, but these too will be susceptible to rupturing and explosion. For larger quantities, a whole new hydrogen pipeline transportation system would need to be constructed or the compressor stations, valves and gaskets of the existing pipeline systems for natural gas will have to be adapted and retrofitted to hydrogen use. This assumes, of course, that the construction material of these existing pipelines will be suited to hydrogen transportation, which may not be the case.
Both reversible and irreversible solid hydrides may be used as sources of hydrogen. The reversible solid hydrides can be used for many cycles. Once their hydrogen supply is depleted the reversible hydrides are able to reabsorb hydrogen and again be used as a hydrogen supply. Reversible hydrides are generally configured to release hydrogen upon being heated to a specified temperature, therefore heat transfer through the reversible hydrides is a controlling factor.
Unlike the reversible hydrides, the irreversible hydrides can only be used for a single cycle. While the irreversible hydrides have a single cycle life, they do not readily dehydrogenate. The hydrogen is generally chemically bonded within the hydride therefore making the hydride extremely stable. These hydrides are preferably made with alkaline or alkaline earth metals due to the reactivity of the alkaline earth metals with water. Water is introduced to the alkaline or alkaline earth metal hydrides and hydrogen gas is readily generated through hydrolysis. The drawback to using irreversible hydrides is that the hydrolysis reaction yields waste products. The waste products must be removed from the system and the hydride must be replenished to maintain a steady supply of hydrogen.
Metal hydrides easily formed by hydrogenation at ambient temperatures are extremely desirable. Most hydrides are only able to absorb their maximum hydrogen capacity at high temperatures which can be costly. A hydride having small crystallite size is desirable as well. To prepare alloys for optimum reactivity, they must first be ground into fine particles thereby increasing the reactive surface area of the alloy and increasing the reaction efficiency. A hydride having a small crystallite size can be easily ground into fine particles without the need for extensive grinding capability. The fine hydride particles provide the reactivity needed to achieve the desired efficiency. Thermal stability is also a main concern whereas many hydrides begin to dehydrogenate at low temperatures. A hydride where the absorbed hydrogen is chemically bonded within the hydride will be extremely useful due to its exceptional thermal stability. The present invention provides a metal hydride that is able to absorb its maximum hydrogen capacity at ambient temperatures. By forming the hydride at ambient temperatures, the resulting hydride has a nano-crystalline structure thereby enabling the hydride to be easily ground into fine particles. The present invention also has exceptional thermal stability in that the hydrogen is chemically bonded within the hydride. The hydride must be exposed to temperatures well above ambient temperatures to even begin to desorb small amounts of hydrogen. This hydride will be found useful in a wide variety of design applications due to exceptional stability, reactivity, and hydrogenation.
The present invention discloses a metal hydride fuel. The metal hydride is composed of lithium, calcium, and hydrogen. Hydrogen within the hydride is chemically bonded to the lithium and calcium to form a stable hydride. Gaseous hydrogen is generated when the metal hydride reacts with water by way of hydrolysis.
The metal hydride has the formula of Ca1+aLi2+bH4+c, wherein a and b are between xe2x88x920.5 and 0.5 and c=2a+b. The metal hydride has a nano-crystalline structure where the crystals have a size less than 100 nanometers. The small crystallite size allows the metal hydride to be easily ground into a fine powder. The metal hydride has excellent hydrogen storage capacity having a maximum hydrogen storage capacity of at least 5 percent by weight.
The metal hydride fuel is produced by first creating a lithium calcium alloy by melting elemental lithium and elemental calcium into a metallic mixture. Elemental lithium and calcium are combined in a crucible and heated to just above the melting point of calcium (839xc2x0 C.) to form a metallic liquid. The metallic liquid is maintained above 300xc2x0 C. for 10 minutes to produce a homogeneous metallic mixture. The lithium and calcium are heated using a induction furnace with an argon atmosphere. The induction furnace operates at 10,000 Hz thereby mixing the metallic mixture. The metallic mixture is then cooled and formed into ingots. The ingots are ground down into an alloy powder and the alloy powder is hydrogenated at ambient temperatures. The rate of hydrogenation of the alloy may be increased by increasing the pressure of the hydrogen gas. Once hydrogenated, the metal hydride is ready for use as a hydrogen fuel in a variety of applications.
One such application may be in an apparatus used to provide a stream of hydrogen gas. In such an apparatus, the metal hydride is placed into a vessel having an inlet and an outlet. The inlet may be adapted to introduce a controlled amount of water to the metal hydride. The water may be in liquid or mist form. The water contacts the metal hydride and gaseous hydrogen is generated. The gaseous hydrogen then travels through the outlet and may be used as a source of fuel. Such an application may be used to provide gaseous hydrogen to a fuel cell used to power a vehicle or distributed and stationary power generation.